Is there a way to enable pressing/releasing shift to Pan while already rotating, without having to release the middle mouse button?
Under User Preferences > Keymap navigate to 3D View > 3D View Global.

Look for the operator Rotate View and under it expand View 3D Rotate Modal.
Add a new entry there with the sub option Switch to Move



You may optionally want to add one to View 3D Move Modal too, for equivalent behavior.